Thieves vs. Samurai: Yeah, Auron + Samurai do about 93% to average HP which Rikku is. So she's getting a turn. Samurais can actually block status somewhat well but Petrify Grenade still kills FF5 at minimum, and if the entire rest of the team is blocking petrify she'll use Sleep Powder instead while Zidane uses... actually he doesn't have a good option against the sleep-immune Samurai. Doom still gives her another turn to launch a second, fatal Zeninage, Silence/Mini don't matter... actually never mind Rikku surely laps Samurai thanks to recharge time and gets off a second move and hits her with petrify. EDIT: Though taking Zeninage as 3x def instead of 2x like I've been doing would flip this the other way, see Snowfire.
Time Mages vs. Summoners: Two of the Summoners can block slow, and that includes FF5... but yeah Haste still matters a lot and if FF5's first turn is Slowga + Graviga that should be enough for FFT or Tidus to kill off the offenders.
Thieves vs. Time Mages: The two thieves which really matter can be hit by Slow, so that gives TMs a lot of time to do whatever nasty things. Rikku certainly isn't getting a turn, I doubt Zidane will either and even if he does, one Zidane turn isn't decisive anyhow.
Thieves vs. Summoners: Petrify Grenade gets everyone except FFT and Rydia, Zidane puts Rydia to sleep. FFT's best bet is probably to wake Rydia up and hope she lives long enough for a status move on Rikku like Death or whatever. Let's say she does. The other two thieves can probably two-round FFT summoner, so they do that, and Zidane then puts Rydia to sleep again, and victory is still collected.
Samurai vs. Time Mages: EDIT: Oh. Hmm, right, Tidus can't actually stop FF5 Samurai + Auron from killing FF5 TM, Delay Attack shouldn't work on someone who is getting a simultaneous turn and FF5's evade + Blade Grasp means that isn't working either. Hastega anyway, drop a space rock... yeah not enough happening here, since FF5 CAN NOT get another turn and almost certainly will due to the anti-physical things + the immunity to slow/stop.
Samurai vs. Summoners: Summoners are kinda frail which makes Zeninage scary... but most of the Summoners are low-HP rather than low-Def so they aren't being MT KOed or anything, and two of the samurais are too slow. FF5 Summoner absolutely must die, and I think she does, but that leaves a nasty MT blitz from Rydia/Garnet/FFT.
Summoners vs. Yuna: Yuna's big weakness here is the lack of MT. She can summon Anima and try to gun down the Summoners one-by-one, but Garnet and FF5 both have revival, so that won't work. (Phoenix is pricy, but three uses is enough). To make matters worse the two non-reviving summoners are the ones with really high damage so I imagine they can do the needed damage (which admittedly is a lot) in the three rounds before Phoenix runs out. As soon as they down an aeon they can win because Rydia and FF5 both have turn-1 status, and since they're ATB they can unquestionably wait for right after FFT or Garnet to do the fatal blow if needed.
That leaves the Magus Sisters. And while they could win... well. Here's the thing. FF5 Odin doesn't check ID immunity, it checks gravity immunity. While I'm not sure how much I would hype that in general, I am quite fine with doing it here, since gravity is a specific weakness the aeons have; there's little question that FF5 Odin would work on them as such. And since only one Magus Sister acts upon being summoned, and is the one who most often behaves badly, and even if she attacks only has a 1/4 chance to hit FF5... yeah an Odin can probably cut the magus sisters apart, leaving Rydia to take out Yuna, or just combined damage from Garnet + FFT.
